Heretofore, in this field, heterojunction bipolar transistor (HBT) fabrication methods have predominately been specific to the application for which the HBT was intended. Low-power circuits, such as digital circuits, require small-area devices to minimize power consumption. High-power circuits, such as power amplifiers, require a maximization of device area without the degradation of device performance. Low-noise devices, such as receivers, on the other hand, require low base resistance and low recombination current operation.
Several HBT fabrication methods have been reported in the past to address these needs, but these methods have always been applicable to only one application at a time. For instance, a fabrication method that produces good high-power devices will usually be unable to produce a good low-noise device, etc. For example, most HBT power amplifiers are fabricated with multiple emitter and base fingers running parallel to each other, as illustrated in FIG. 1. The finger dimensions and finger-to-finger spacings are determined from power gain and power output (thermal) considerations. In the example shown in FIG. 1, all emitter fingers 10 and base fingers 12 are connected to each other on semi-insulating substrate 14. The device active area 16 is defined by ion implantation. Although this approach is well suited to power device fabrication, it has several disadvantages when used to fabricate low-power and low-noise devices. These include:
1) The active area is determined by ion implantation. This enhances recombination currents at the emitter-base junction which increase 1/f noise and lower current gain.
2) It is difficult to define very small device areas with this method (e.g. emitter finger length cannot be much smaller than 3-5 microns).
3) Since the base current is injected only from the edges of the emitter area, the base series resistance due to the section of base under the emitter contact remains high. A reduction of this resistance is only possible by reducing the finger width, which in turn reduces power density.
4) The emitter finger length is limited by current density limitations in the emitter metal. Since all emitter current is supplied from one end of the emitter finger, electromigration current limits in the emitter contact metal are exceeded for fingers longer than 30 microns and emitter metal thicknesses of 0.6-1.0 micron.
As the HBT technology matures, it is becoming apparent that high-speed (i.e. microwave) HBT circuits will be required to perform multiple functions within a single integrated circuit. An example of this would be a transceiver chip in a radar or communication system. This would require the fabrication of low-noise, low-power devices for the receiver section simultaneously with power devices for the transmitter section.
Accordingly, improvements in HBT fabrication techniques which overcome any or all of the limitations of the present techniques are clearly desirable.